The present invention relates to filters for use in exhaust streams for capturing particulate material. In particular the present invention relates to porous ceramic diesel exhaust filters which can be regenerated by microwave energy.
Recently much interest has been directed towards the diesel engine due to its efficiency, durability and economical aspects. However, diesel emissions have come under attack both in the United States and Europe, for their harmful effects on the environment and on humans. As such, stricter environmental regulations will require diesel engines to be held to the same standards as gasoline engines. Therefore, diesel engine manufacturers and emission-control companies are working to achieve a diesel engine which is faster, cleaner and meets the most stringent of requirements under all operating conditions with minimal cost to the consumer.
One of the biggest challenges in lowering diesel emissions is controlling the levels of diesel particulate material (DPM) present in the diesel exhaust stream. In 1998 DPM was declared a toxic air contaminant by the California Air Resources Board. As mentioned herein above legislation has been passed that regulates the concentration and particle size of DPM pollution originating from both mobile and stationary sources.
DPM which is mainly carbon particulates, is also known as soot. One way of removing diesel soot from the diesel exhaust is through diesel traps. The most widely used diesel trap is the diesel particulate filter (DPF) which is used to capture the soot. The DPF is designed to provide for nearly complete filtration of the soot without hindering the exhaust flow. However, as diesel soot accumulates, exhaust flow becomes increasingly difficult and the DPF must either be replaced or the accumulated diesel soot must be cleaned out. Cleaning the accumulated diesel soot from the DPF is achieved via burning-off or oxidation to CO.sub.2 and is known in the art as regeneration. Regeneration is considered to be a superior approach over DPF replacement since no interruption for service is necessary.
The regeneration process can be either passive or active. In a passive system, regeneration occurs when the DPF becomes so filled with carbon particulates that heat accumulated in the exhaust system due to excessive back pressure raises the temperature of the carbon to a point where it ignites. This design can result in thermal shock or melt down of the filter, high fuel penalty and poor filtering action. Active regeneration is considered to be a superior approach over passive regeneration. In an active system, heat required to initiate combustion of the soot is generated by an outside source. Electrical power, fuel burners and microwave energy have all been studied as heat sources. Microwave energy is considered to be a superior approach over electrical power and fuel burners because it is highly efficient cost- and energy-wise.
Microwave regeneration has been addressed, for example in U.S. Pat. No. 5,087,272 (Nixdorf) which discloses a microwave regenerated filter made of single crystal silicon carbide whiskers which are consolidated into a preform of cylindrical configuration or into a thin layer such as a paper, which is then folded into a multicellular form. A problem associated with the proposed filter is that it is labor intensive and time consuming to manufacture, and hence not adaptable to high efficiency production methods.
Standard commercially available filters are made of cordierite (2MgO-2Al.sub.2 O.sub.3 -5SiO.sub.2). However, cordierite is transparent to microwaves and is not regenerable upon exposure to microwave energy.
Therefore a need exists for a filter for trapping and combusting particulates from a diesel exhaust stream which can undergo regeneration by microwave energy and which can be manufactured according to high efficiency production methods (i.e., extrusion), while at the same time exhibiting a high filtration efficiency.
It is the purpose of the present invention to provide such a filter.